1. Field of the Invention
The invention relates to a method for starting the operation of a device for providing a liquid additive.
2. Related Art
Devices for providing a liquid additive are used, for example, in the automotive field for supplying a liquid additive to an exhaust-gas treatment device for purification of the exhaust gases of an internal combustion engine of the motor vehicle. Exhaust-gas treatment devices in which a liquid additive is used for the purification of exhaust gases are widely used. An exhaust-gas purification method particularly commonly implemented in such exhaust-gas treatment devices is the method of selective catalytic reduction (SCR method). In this method, nitrogen oxide compounds in the exhaust gas are reduced with the aid of a reducing agent. Here, ammonia is typically used as reducing agent. The exhaust-gas treatment device typically has an SCR catalytic converter on which the nitrogen oxide compounds in the exhaust gas are reduced with the aid of the ammonia. Ammonia is usually stored in motor vehicles not directly but rather in the form of a reducing agent precursor solution. The reducing agent precursor solution is a liquid additive. One reducing agent precursor solution that is particularly frequently used is urea-water solution. A 32.5% urea-water solution is available under the trade name AdBlue®.
Upon the start of operation of a device of said type, it is a problem that the liquid additives can freeze at low temperatures. The urea-water solution described above, for example, freezes at −11° C. Such low temperatures may be encountered in particular during a long standstill period of the motor vehicle. After a long standstill period, it may be the case that the liquid additive in the device has frozen completely. The device then initially cannot provide any liquid additive. It is known for devices for providing liquid additive to have a heating system for melting frozen liquid additive, such that a provision of liquid additive is possible promptly after a start of operation.
As heating devices for such devices, PTC (positive temperature coefficient) heating elements are proposed in particular. PTC heating elements are electrical heating elements heated by an electrical current flowing through them. They have the additional characteristic that the electrical resistance for the current increases with rising temperature. It is thus achieved that the electrical current automatically decreases at high temperatures. As a result of the decrease of the electrical current, the heating power also decreases. This provides automatic protection of a PTC heating element against overheating.
However, in the case of such devices being heated by PTC heating elements, it is a problem that there is overall only a limited available amount of current, which is defined by a maximum power of a current supply source, for the operation of the device. In particular during the start of operation (when a supply must be provided to a multiplicity of current consumers simultaneously), the amount of current may not be sufficient to correctly maintain all functions of the device. The current consumption of PTC heating elements during the start of operation is a particular problem.